1. Field of the Invention
The present invention relates to dense plasma focus radiation sources for the production of extreme ultraviolet radiation, useful for lithography, and more particularly, to methods of Lithium delivery into the plasma, to methods of heat removal from the electrodes, to methods of improvement of lifetime of electrodes, to methods of improvement uniformity of the discharge and to methods and apparatuses for protecting the optics within the plasma radiation source, from the plasma.
2. Discussion of Related Art
Features on semiconductor wafers may be produced using lithography techniques. At present deep ultraviolet (DUV) lithography techniques use DUV lasers having a wavelength of the order of 200 nanometers (nm) which can be used to print sizes of about 100 nm. However, as semiconductor technology improves and the size of semiconductor devices decreases, it may be desirable to be able to print features with a size of the order of 70 nm and smaller. In order to print features of this size, a lithography light source with a smaller wavelength than DUV is needed. Presently, a light source of choice is in the extreme ultraviolet (EUV) having a wavelength of about 13.5 nm.
Plasma devices that may be used as radiation generators comprise of a center electrode and an outer electrode substantially coaxial with the center electrode, with a coaxial column being formed between the electrodes. FIG. 1 illustrates an example of a discharge chamber portion of a high repetition rate dense plasma focus (DPF) radiation source with a coaxial electrode geometry. A selected gas, for example, a mixture of noble gases or mixture of noble gas with Lithium or Tin vapor, may be introduced into a column 26 through an inlet mechanism (not illustrated) to provide the plasma medium. The plasma is initially ignited over the surface of the insulator 42 that electrically isolates the anode 46 and the cathode 44 and is subsequently driven as a plasma sheath off the insulator. The outer electrode 44 (the cathode) may be solid (as illustrated) or may be a plurality of substantially evenly spaced rods arranged in a circle (not illustrated). A high repetition rate pulse driver 28 is provided at a base of column 26. The driver 28 is adapted to deliver a high voltage pulse across the electrodes. The plasma sheath is driven by magnetic forces from the base end of the column to the end of the column where the plasma sheath pinches near the top of the center electrode. The magnetic forces resulting from the current flowing between the electrodes drive the plasma sheath up the coaxial column 26. Once the plasma sheath has reached the tip of the anode 46, magnetic forces drive the plasma radially inward, causing a pinch 30 with a radially trapped slug of ionized gas that reaches a high temperature. The amplitude of the pulse voltage, the current rise time of the pulse, the gas pressure and the electrode lengths may be selected so that the current for each discharge pulse is substantially at its maximum as the plasma pinches. The high temperature plasma in the pinch produces radiation 32 in the desired EUV band centered around 13.5 nm. The amplitude of the EUV radiation and it's wavelength are controlled through careful selection of various device and plasma parameters including the selected gas fed to the pinch, the amplitude and pulse length of the current produced from the driver 28, the plasma temperature in the area of the pinch, and the gas pressure in the column. This type of DPF device can also be designed to radiate at shorter wavelengths. However for shorter wavelength radiation, a higher plasma temperature is desired. Some examples of plasma guns are described in U.S. Pat. No. 6,300,720 to Birx, issued Oct. 9, 2001, which is herein incorporated by reference in its entirety.